As bearings subject to higher surface pressure and lower speed conditions (such as bushings for implements mounted on construction machines), steel bushings, which have been carburized or induction-hardened to attain wear resistance as an important feature, are presently used in grease-lubricated situations. Since the lubricating conditions, in which such implements for construction machines are used under high surface pressure, are particularly harsh, unpleasant abnormal noises occur when the implements are in service. As measures for avoiding noises, a lubricating film is applied to the sliding contact surfaces of the steel bushings, or a multiplicity of grease grooves are made for promotion of grease lubrication.
Oil retaining sintered bearings made from iron base (typically Fe—C—Cu base) sintered contact materials are sometimes used in part of implements subject to low load. These iron base sintered contact materials include a hard martensitic matrix and pores formed in the matrix, the pores being impregnated with a lubricating oil. Some of them further contain softer tool powder or ceramic powder.
As copper base sintered bearing materials, bronze base materials such as Cu—Sn—Pb and lead-bronze base materials are commonly used in the roller section of the base carrier of a construction machine. On the other hand, high strength brass bushings, which are harder and stronger than these materials, are employed in part of implements, thanks to their excellent seizure resistance and conformity.
In order to extend greasing time intervals at which grease is fed to the bearing section of an implement, bearings such as “500SP” produced by OILES CORPORATION. are used in some cases. In such bearings, a high strength brass bushing is provided with machining holes whose area is about 30% of the sliding contact area and these holes are so arranged as to be overlapped with one another in a sliding direction and filled with graphite as a solid lubricant. For the same purpose, sintered metal bodies such as “SL Alloy” produced by TOSHIBA TUNGALOY CO., LTD., to which a large amount of solid lubricant is added, are sometimes used.
A double-layered sintered contact component used under high surface pressure conditions and a producing method thereof are disclosed in Japanese Patent Publication (KOKAI) No. 5-156388 (1993). This double-layered sintered contact material contains graphite as a solid lubricant within the range of from 3 to 8 wt % and comprises an aluminum-bronze base sintered contact alloy which is integrally bonded to a steel plate with a joint layer of phosphor-bronze plate therebetween and which contains 5 to 13 wt % Al, 3 to 6 wt % Fe and 0.1 to 1.5 wt % TiH.
Lubricating film forming conditions for contact components that slide under high surface pressure at extremely slow speeds like bushings used for the implements of working machines are extremely severe. The above steel bushings are hard enough to withstand high load, but present the serious drawback that they are susceptible to seizure and unpleasant abnormal noises so that control is needed to prevent seizure and noises by shortening greasing time intervals.
The above implement bushings formed from the oil-retaining iron base sintered contact materials having a martensitic matrix do not fatigue and are improved over the steel bushings in terms of seizure resistance. However, they suffer from the problem that when they are used under extremely slow and high load conditions such as encountered by implements, lubricant starvation tends to occur and as a result, satisfactorily improved seizure resistance and prevention of abnormal noises cannot be ensured.
The contact components made from the sintered contact materials, which are provided with pores impregnated with a large amount of lubricant for improving the lubricating condition during sliding, have also failed in improving seizure resistance and prevention of abnormal noises as much as expected, because the lubricating condition gets all the worse for the provision of a number of pores within the sintered body.
Where a bronze base material (e.g., Cu—Sn and Cu—Pb), which is a material composed of dissimilar constituents, is used with the intention of increasing resistance to seizure caused between an implement steel pin and an implement bushing, the material becomes fatigued under high surface pressure and tends to wear away very soon because of the severe lubricating condition.
Where a cast high-strength brass material, which is harder and stronger than the bronze base material, is applied for an implement bushing, substantially no fatigue is caused and occurrence of abnormal noises can be prevented to a considerable extent compared to the case of the steel bushings. However, lubricant starvation easily occurs as pointed out earlier so that satisfactory improvements in seizure resistance and protection against abnormal noises cannot be expected.
In cases where graphite having high self-lubricity is embedded in a cast high-strength brass bushing and the graphite is impregnated with a lubricating oil for achieving increased greasing ability and prolonged greasing intervals, the area percentage of the holes to be filled with graphite is normally limited to 25 to 30%, so that the area over which the lubricant is spread decreases as the sliding distance decreases, resulting in occurrence of local seizure and a failure to ensure satisfactory self-lubricity for a long time. In addition, the processes of machining to make the holes for graphite and filling the holes with graphite lead to a considerable increase in the cost.
Further, the technique such as disclosed in Japanese Patent Publication No. 5-156388, in which graphite is added as a solid lubricant to a high-strength aluminum bronze sintered material in a large amount of 3 to 8 wt % (about 12 to 36% by volume) in order to ensure improved seizure resistance, has not proved successful in that brittleness due to the large graphite content leads to poor sliding properties under high surface pressure and insufficient wear resistance.
Metallic sintered bodies containing large amounts of solid lubricant are difficult to be sintered and therefore require pressurization treatment during sintering to achieve practical strength. For instance, in the case of the above-described double-layered sintered contact component formed by integrally bonding an Al bronze base sintered contact material containing 3 to 8 wt % graphite to a backing with a phosphor bronze material therebetween, pressurization treatment is needed in the sintering process and at least the integration process inevitably causes an increase in the cost.
The present invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a contact material which provides improved wear resistance as well as reduced adhesion by virtue of the features of an intermetallic compound having an ordered phase, with the intention of (i) improving the seizure resistance and/or wear resistance of an implement bearing which is subject to low-speed, high-surface-pressure conditions when sliding and is susceptible to lubricant starvation; (ii) preventing abnormal noises; and (iii) achieving prolonged greasing intervals. Another object of the invention is to provide a composite sintered contact component in which the above-described contact material is integrated with a backing and a method of making the same.